The present invention relates to a drive transmission apparatus for vehicle operative to distribute a drive force between front and rear drive wheels or between left and right drive wheels, as well as to a four-wheel drive vehicle.
There is known a permanent four-wheel drive vehicle wherein a drive transmission apparatus is disposed on a drive-force transmission path between the front and rear wheels for varying a distribution ratio of the drive force between the front and rear wheels according to a running state of the vehicle, thereby varying torques transmitted to the front and rear wheels.
There is also proposed a four-wheel drive vehicle which further includes a drive transmission apparatus disposed on a drive-force transmission path between the right and left wheels so that the distribution ratio of the drive force is varied not only between the front and rear wheels but also between the right and left wheels.
A multiple disc clutch is employed as a device for transmitting the drive force to the front and rear wheels or to the right and left wheels. The multiple disc clutch includes an electromagnetic type adapted to switch ON or OFF the drive transmission based on an electromagnetic force, and a hydraulic type adapted to switch ON or OFF the drive transmission based on a hydraulic pressure.
In comparison between the electromagnetic type and the hydraulic type, the electromagnetic type is inferior in clutching feeling because the operation mechanism thereof involves the occurrence of speed change shock at each switch-on or switch-off of the clutch. On the other hand, the hydraulic type has a superior clutching feeling than the electromagnetic type.
Therefore, the use of the hydraulic type is recommended from the viewpoint of clutching feeling.
Where the multiple disc clutch is of the hydraulic type, an oil-hydraulic circuit is necessary. Conventionally, a feed oil to the multiple disc clutch is drawn from a transmission or the like, whereas an oil pump for the oil-hydraulic circuit is driven by an engine.
Where the oil pump is driven by the engine, however, the oil pump produces an additional load on the engine, resulting in a poor fuel economy.
On this account, the oil pump is desired to be an electric pump driven by an electric motor rather than by the engine.
Unfortunately, the electric motor employed by the conventional electric pump is a brush motor, which is detrimentally vulnerable to high temperatures. In a case where, for example, the brush motor is located in the vicinity of the multiple disc clutch, the brush motor is exposed to a high-temperature environment because the multiple disc clutch is located in the proximity of the transmission, a differential gear and the like, which are heated to 120° C. or more. In the high-temperature environment, the brush motor suffers serious brush wear so as to be significantly reduced in the service life thereof when it is rotated continuously.
In order to avoid the brush wear, it may be contemplated to employ an electric brushless motor with a sensor. The brushless motor with a sensor is a brushless motor equipped with a sensor for sensing an angular position of a rotor.
However, the brushless motor with a sensor is still unsuited for use in the high-temperature environment because the sensor has poor high-temperature durability.
Thus, every type of motor is vulnerable to the high-temperature environment. Accordingly, where the electric pump is employed as the oil pump serving as a hydraulic pressure source for operating the multiple disc clutch, the electric motor must be located at place remote from the multiple disc clutch and of low temperatures, rather than in the vicinity of the multiple disc clutch located in the high-temperature area.
However, such a layout requires an oil piping for interconnecting the multiple disc clutch and the electric motor, the oil piping requiring an extra space therefor. Consequently, the drive transmission apparatus has a lower degree of freedom for layout and also suffers a reduced efficiency because of an increased pressure loss associated with the oil piping.